1. Field of the Invention
The present invention relates to a supercharger with an electric motor, which has an electric motor assisting a rotational drive of a compressor and is driven by an exhaust gas of an internal combustion engine and compressing an intake air so as to supercharge.
2. Description of Related Art
In order to improve a performance of an internal combustion engine, there has been widely used a supercharger (also called as “turbocharger”) driven by an exhaust gas of an internal combustion engine and compressing an intake air so as to supercharge. Further, there has been employed a supercharger with an electric motor in which an acceleration response or the like is improved by installing the electric motor on the same axis as a shaft of the supercharger and assisting a rotational drive of a compressor in an accelerated manner.
The following patent document 1 discloses a prior art relating to the supercharger with the electric motor. FIG. 1 is a cross sectional view showing a structure of a supercharger 50 with an electric motor disclosed in the patent document 1. In an inner portion of a housing 51 of the supercharger 50, a turbine impeller 52 is arranged in an exhaust passage side, a compressor impeller 53 is arranged in an intake passage side, and the turbine impeller 52 and the compressor impeller 53 are coupled by a shaft 54. The shaft 54 is rotatably supported by a bearing 55 installed in the housing 51. Further, in the housing 51, there is installed an electric motor 58 having a rotor 56 coaxially coupled to the shaft 54, and a stator 57 arranged around the rotor 56. In the supercharger 50 structured as mentioned above, the turbine impeller 52 is rotated by an exhaust gas from an internal combustion engine, the compressor impeller 53 coupled to the turbine impeller is rotationally driven, the rotational drive is assisted by the electric motor 58, and an intake is supercharged so as to be supplied to the internal combustion engine.
In the supercharger with the electric motor mentioned above, the electric motor 58 is rotated at a high speed during the operation of the supercharger, and is self-heated due to a windage loss and an eddy current loss. Further, since a high-temperature exhaust gas flows through the turbine, the electric motor 58 comes to a high temperature on the basis of a heat conduction from the turbine impeller 52 to the shaft 54, and from the shaft 54 to the rotor 56 of the electric motor 58. When the electric motor 58 comes to the high temperature, there are generated problems that an internal permanent magnet is demagnetized and an efficiency of the electric motor 58 is lowered. Accordingly, the supercharger in the patent document 1 is structured, as shown in FIG. 1, such that a cooling fluid flow path 60 is formed in an inner portion of a housing 51 so as to surround the electric motor 58, thereby flowing a cooling fluid 61 through the cooling fluid flow path 60 so as to cool the electric motor 58.
Further, the other prior art relating to the cooling of the electric motor in the supercharger with the electric motor is also disclosed in the following patent document 2. The supercharger with the electric motor disclosed in the patent document 2 is provided with a cooling gas introduction path 72 for introducing a part of an air in an outlet of an intercooler 71 to an inner portion of a housing 73 of a supercharger 70, and is structured such as to cool the electric motor 74 by the cooling air introduced to the inner portion of the housing 73, as shown in FIG. 2.
[Patent Document 1]
Japanese Unexamined Patent Publication No. 2003-293785
[Patent Document 2]
Japanese Unexamined Patent Publication No. 2005-69178
In the cooling structure in the patent document 1 mentioned above, it is possible to cool the stator 56 side of the electric motor 58, however, since the rotor 56 is arranged at the predetermined interval from the stator 57, it is impossible to sufficiently achieve the heat conduction to the rotor 56 side. Accordingly, it is impossible to sufficiently cool the rotor of the electric motor, and there remain the problems that the permanent magnet is demagnetized and the efficiency of the electric motor is lowered. Further, in order to solve the problem, there is considered a method of introducing the cooling air from the outlet of the intercooler 71 to the electric motor 70 as in the cooling structure shown in the patent document 2 mentioned above. However, in accordance with this method, since it is necessary to add an external piping such as the cooling gas introduction path 72, there is a problem that a weight and a cost of the supercharger itself are increased.